1. Field of the Invention
The present invention relates to a pellicle used as the dust-proof cover during exposure of a photomask or a reticle.
2. Description of the Related Art
At the lithographing step, a mask comprising a glass sheet and a circuit pattern of a vacuum deposition film of chromium or the like (hereinafter called mask) formed on the surface of the glass sheet is used, and the circuit pattern is transferred to a resist-coated silicon wafer. At this step, if light exposure is carried out when a foreign substance such as dust is adhered to the circuit pattern on the mask, the foreign substance is transferred to the wafer and a defective product is formed. Especially, when light exposure is carried out by using a stepper, the risk that all of the chips formed on a wafer will be defective is increased, and an adhesion of foreign matter to a circuit pattern of a mask or the like becomes a serious problem. Therefore, a pellicle has recently been developed and used by which this problem is eliminated.
In general, the pellicle comprises a transparent film of nitrocellulose or the like spread on one side face of a pellicle frame made of aluminum, a double-adhesive tape is applied to the other side face of the pellicle, and the pellicle is attached to a mask. When this pellicle is used, the intrusion of a foreign substance from the outside can be prevented, and even if a foreign substance adheres to the film, the foreign substance is transferred in the fuzzy state during light exposure and no particular problem arises. Nevertheless, the foreign substance already attached on the innerside of the film or the pellicle frame can no longer be removed, and when such a pellicle is used, to prevent interference with the exposure because foreign matter attached to the innerside of the pellicle has fallen onto the photomask or the reticle, a pellicle provided with a tacky film on the inner side face of the pellicle frame has been proposed (see: Japanese Unexamined Pat. Publication (Kokai) No. 60-57841).
On the other hand, a single layer thin film of nitrocellulose has been primarily utilized as the pellicle film, and to improve the stability of the light transmittance and the like during the exposure step, a pellicle film having a reflection preventive layer comprising a fluorine type polymer or a silicon type polymer and the like on a transparent film of nitrocellulose has been proposed (see: Japanese Unexamined Pat. Publication (Kokai) No. 60-237450).
Nevertheless, in the pellicle described above (see: Japanese Unexamined Pat. Publication (Kokai) No. 60-57841), since a double-adhesive tape is used at the adhered face between the pellicle frame and the mask, a foreign substance such as dust is generated from the foamed tape which is the substrate for the double-adhesive tape, and becomes a source of contamination.
Accordingly, Japanese Pat. application No. 62-205533 proposed a pellicle which does not use a double-adhesive tape at the adhered surface. But, even in this method, the foreign matter attached on the inner face of the pellicle film may be sometimes caused to fall onto the mask due to shock during usage, to thereby cause the production of defective LSI's.
On the other hand, the pellicle film used as a dust-proof film must have a high light ray transmittance, and accordingly, the presence of a foreign substance is extremely disadvantageous, and thus a material to which the attachment of a foreign substance is difficult has been used. When forming a reflection preventive film as disclosed in the above-mentioned Japanese Unexamined Pat. Publication (Kokai) No. 60-237450, however, a material without tackiness on the surface must be used to prevent the attachment of a foreign substance. Nevertheless, although such a material without tackiness on the surface can form a film on which a foreign substance can be attached only with difficulty, any foreign substance attached thereto will readily drop therefrom, and thus the problem arises that such a foreign substance will interfere with the exposure.